I'll give you two answers, my own and a more professional one.
My own opinion: Bark collars suck, plain and simple, LOL. They're either cruel (shock collars) or they are terribly ineffective (spray collars).
A more professional answer: Quoted from an interesting article I found on UltimateHomeLife
"This method is one that youâ€™ve probably heard of before. Why? Because itâ€™s wildly popular.
Whenever your dog barks, bark collars automatically catch on and emit something that interrupt your dog from barking. Depending on the bark collar you get, the interrupter might be very different for you than for someone else.
Some bark collars emit a noise, while others make use of a small frequency of electric stimulation. There are many other methods bark collars use that are way too many to list in this article, so the choice is up to you. Explore your options and see which one youâ€™re most comfortable using on your dog.
However, one important word on shock collars. By delivering a certain electric frequency to your dog, this can be painful for them. And when they feel pain, this can cause both physical and psychological pain and trauma, and may make your dog very aggressive, especially if they are able to tell where the source of the pain is coming from (where theyâ€™ll become aggressive towards you if theyâ€™re able to tell youâ€™re behind the electric frequency).
As for collars that squirt a scent of citronella when your dog barks, your dog knows how to adapt to that faster than you think. Believe it or not, your dog can catch on to what the can is there for, manipulate it and use it up real fast and start barking at will again."